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Styrene Polymerization Catalyzed By Rare Earth Catalyst System And Ultrahigh Molecular Weight Polystyrene

Posted on:2002-12-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:L B WuFull Text:PDF
GTID:1101360032955054Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
Ultrahigh molecular weight polystyrene (UHMWPS), as a novel styrene polymer, is characterized by higher impact resistance. Styrene polymerization catalyzed by rare earth catalyst system, first discovered by Professor Shen Zhiquan's group, is a new method to synthesize UHMWPS. However, research work on some key academic and industrial problems, such as formation process and structure of catalytic active center, polymerization kinetics and mechanism, suitable polymerization process, rheology and mechanical properties of UHMWPS still remain to be done.In this work, a ternary rare earth catalyst system neodymium phosphate / magnesium aluminum alkyl compound / hexamethylphosphoramide (Nd(P204)3/MgBu2-AlEt3/HMPA) is used to initiate styrene polymerization and UHMWPS with molecular weight up to 2 x 106g-mor' is successfully synthesized. The preparation methodology of homogeneous catalyst system, formation process and structure of catalytic active center, polymerization kinetics and mechanism, kinetic modeling, polymerization process, stereo-regularity, rheology and processability, and properties of UHMWPS have been investigated systematically.First, the ternary catalyst system Nd(P204)3/MgBu2-AlEt3/HMPA is prepared by three different methods and the effects of aging time, aging temperature and alkylation reaction on polymerization conversion, molecular weight of polymer and number of polymer chains have been investigated. A feasible and reasonable procedure is obtained to prepare homogeneous catalyst systems. The catalyst system is very stable at ambient temperature and its stability is enhanced by addition of small amount of styrene monomer during preparation. The violet-visible spectra of the catalyst solution indicates that the rare earth metal Nd remains +3 valence in thepreparation process. According to the change of the properties of the catalyst system during its preparation process and its effect on polymerization, the mechanism of the formation of the catalytic active center and its structure are suggested.The isothermal kinetics of the bulk polymerization of styrene initiated by the ternary homogeneous catalyst system Nd^oOs/MgBu^AlEtj/HMPA under various reaction conditions is investigated. An auto-accelearation phenomenon on polymerization rate has been found even when the polymerization temperature is well controlled; and the molecular weight of the resultant polystyrene increases linearly with the monomer conversion but does not passes through the origin. The final molecular weight reaches up to 2X106g-mol"', the molecular weight distributions are relatively narrow and the polydispersity is between 1.5 and 2.0. Polymerization proceeds until all of the monomer has been consumed and further addition of monomer results in continued polymerization. Based on theoretical analysis on the experimental results, a mechanism for this polymerization system is suggested first, which is slow initiation, rapid propagation and transfer to the magnesium aluminum alkyl compound.Base on the mechanism mentioned above, a kinetic model containing diffusion limitation of propagation rate constant is developed for the bulk polymerization of styrene catalyzed by Nd(P2(M)3/MgBu2-AlEt3/HMPA, and the kinetic parameters are estimated. The calculated conversion and average molecular weight of polymers by the model agree well with the experimental data. The auto-acceleration phenomena and the linear increase (but not passing through the origin) of molecular weight of polymers are successively interpreted by the model. The model may be feasible to be extended to describe other rare earth catalyzed polymerizations with such kinetic characteristics. According to the model, a kinetic criterion for slow initiation (/.昿/>103'4) is presented, and the complicated molecular weight developments with conversion in rare earth catalyzed polymerization reported in literatures can be interpreted by the degree ofchain transfer reaction.In order to develop a feasible and reasonable process to carry out styrene polymeriza...
Keywords/Search Tags:polystyrene, ultrahigh molecular weight, rare earth catalyst, polymerization kinetics and mechanism, kinetic model, rheology, structure and properties
PDF Full Text Request
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